When a telephone is off the hook, access to the phone by outside callers is generally prevented. In most phone systems, after the phone has been off-hook for a period of time and inactive, the phone company typically sends a warning signal that can be heard through the receiver.
Shapiro, U.S. Pat. No. 4,686,697, discloses a telephone checking system which generates and detects an off-hook load to test the condition of telephone lines. A microprocessor system periodically seizes a line and causes the system to go off hook. If the off hook load is detected by the microprocessor control system over a line, the telephone is assumed to be in good status. Thereafter, the phone is returned to normal operation. If, however, the off hook load is not detected, the microprocessor control system generates a bad line signal that is used to cause an LED pilot light to blink.
Bartelink, U.S. Pat. No. 4,390,750, discloses a remote supervisory system in which sensors are coupled to a subscriber s telephone circuit. If an abnormal condition occurs, the condition is sensed by one of the sensors and the equipment creates a temporary off-hook condition on the circuit, transmits alarm report signals indicative of the abnormality to a central office and then goes back on-hook.
Fahey, U.S. Pat. No. 4,273,960, discloses an apparatus for monitoring usage of a telephone which is connected across tip and ring terminals of the telephone system. The circuitry, under microprocessor control, repetitively samples the voltage across the tip and ring terminals to identify the transitions between on-hook and off-hook. For each detected transition an activity timer within the microprocessor counts the time period until the next transition. In the event the activity timer times out, an alarm condition is produced.
Korwin, U.S. Pat. No. 4,230,914, discloses a telephone intruder alarm system which indicates to an on-line user of a primary extension telephone, a subsequent connection and disconnection of another, auxiliary extension telephone within the same telephone line. Electrical off- and on-hook signals are generated each time an extension telephone is interconnected or disconnected. Such signals are detected and used to illuminate a light emitting diode thereby indicating when another phone has been taken off-hook.